Miscellaneous Structures Stilling Basin Stilling Basin Stilling basin is used at the end of an Ogee type weir to reduce the velocity of the running water down the weir to prevent scour to the downstream channel. Stilling basin is used at the end of an Ogee type weir to reduce the velocity of the running water down the weir to prevent scour to the downstream channel.

d2d2 d1d1 H Z

To design the stilling basin, one must calculate the exit velocity, i.e. the velocity at the foot of the basin, the water depth and the Froude number. Following the illustration in the previous page, To design the stilling basin, one must calculate the exit velocity, i.e. the velocity at the foot of the basin, the water depth and the Froude number. Following the illustration in the previous page, V = C v √(2 g Z ) d 1 = Q/(VB) d 1 = Q/(VB) F = V/√(g d 1 )

where C v is the velocity coefficient (0.97) B is the spillway width Based on the Froude number, one may select the suitable basin type. Based on the Froude number, one may select the suitable basin type. Froude No. Type 2.5 – 4 IV > 4 V 4 V< 20 m/s III > 4, V> 20 m/s II

Example: Example: Design a stilling basin based on the following data; Z = 15 m; Q = 50 m 3 /sec; B =20 m Sol. Sol. V = 0.97 √(2*9.81*15) = m/sec d 1 = 50/(16.64*20) = 0.15 m F = 16.64/√(9.81*0.15)= 13.7 Then select type III

from curve (b) T.W. depth = d 2 T.W. depth/ d 1 = 18.5 T.W. depth d 2 = 18.5*0.15 = 2.78 m from curve (c) h 3 /d 1 = 3 ; h 4 /d 1 = 1.8 Baffle block height h 3 =3*0.15=0.45 m End sill height h 4 = 1.8*0.15 = 0.27 m from curve (d) L/d 2 = 2.8 Stilling basin length L = 2.8*2.78 ≈ 8 m

H 4 =0.27 m H 3 = 0.45 m 0.2 h 3 = 0.09 m h 1 =d 1 =0.15 m L = 8 m